change of employment application, reference number

I have tier 2 visa which was issued outside of UK, now I am changing employers in UK for the first time.

The application form for change of employment is asking me for a Home office reference number but i never had correspondence with them here. I had my visa refused before and I have stamps on my passport from the refusals, would that be my home office reference number?

and can someone tell me please what documents i need to take with me other than my passport and the CoS. Do I ned to rake my degree, IELTS, or any previous documents that I have already supplied to get the Tier 2 in home country or just the application form and CoS should be fine?

Thanks anyone who can reply.